1. Field of the Invention
This invention relates to a process of pulse duration modulation for improved voltage and frequency efficiency of a multi-phase pulse forming converter.
2. Description of the Prior Art
Processes for pulse duration modulation are known and are based on modulating the duration of the output voltage pulses of each converter phase with the desired time dependent voltage corresponding to the time dependent voltage of the appropriate load phase. The frequency spectrum of the output voltage of the converter comprises a modulating time dependent component, possibly higher harmonics thereof and additional waves having frequencies consisting of integral linear combinations of the modulating frequency and of the so-called carrier frequency determined by the pulse number and pulse type of the output voltage.
With increasing multiplexity of the modulating frequency and of the carrier frequency in general the importance of the waves decreases as a result of smaller amplitudes and larger frequencies. There are two disadvantages connected with this method. The modulated amplitude of the desired time dependent voltage is limited to a value corresponding to the merging of two neighboring pulses in the output voltage of the converter. Depending on the circuitry of the converter, on the number of the independently controllable switching elements in one phase and on a definite concrete type of pulse duration modulation, there is a limit to the applicable minimal value of the ratior carrier frequency and modulating frequency. In addition there is a limit to the frequencies for which the converter is useful. Sometimes it is unavoidable to select a disadvantageously high frequency of the switching processes in the converter for a certain frequency of the converter output voltage.